1. Field of the Invention
The subject invention relates to circuitry for rank ordering a set of numbers, and particularly to such a circuit having the capability to adjust rank in a single clock cycle, while eliminating the effect of a sample or test cell on the number ranking and providing a sample having a particular selected rank as an output. The circuit is particularly applicable to radars which employ ordered statistic (OS) constant false alarm rate (CFAR) detection circuits, but has broad applications in many other number ranking contexts.
2. Brief Description of Related Art
A new method of computing thresholds for a radar detector is described in "Radar CFAR Thresholding in Clutter and Multiple Target Situations" by Herman Rohling in IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-19, No. 4, July 1983. This method offers several important advantages over presently-used CFAR methods. Such methods use a CFAR circuit which computes a mean level based on the arithmetic average of the video in the area around the sample of interest. As explained by Rohling, this cell averaging method does not perform well in the presence of clutter or multiple targets. An Ordered Statistic (OS) CFAR can provide performance which is nearly equal to the cell average method when in a noise only environment, and it provides improved performance when operating in an environment containing clutter and/or multiple targets.
An OS CFAR Detector performs the following functions:
1. Inputs all video samples within N/2 samples on either side of the sample cell which is being examined for the presence or absence of a Target. PA1 2. Examines the amplitude of the N samples and assigns each of them a rank according to relative amplitude. This is analogous to sorting N different numbers in order by the values of the numbers. PA1 3. Selects the sample which has a Predetermined Rank (RT) from the sample set. The amplitude of this sample is used as a basis for setting the Target Detection Threshold. PA1 4. Determines the number of samples (Nc) which exceeds the average noise level by a predetermined amount. When this number exceeds certain predetermined values the circuit will adjust RT to prevent an increase of the false alarm rate in severe clutter. As RT changes the circuit automatically adjusts the detection Threshold offset (TD) to maintain the desired probability of false alarm.
All of the above functions must be performed for each video sample point and in a parallel fashion such that the detection function can take place in real time at a rate that matches the video bandwidth.
As may be appreciated, an essential feature of an OS CFAR detector is the ability to rank order a set of numbers. Prior methods used for rank ordering a set of numbers involved algorithms implemented on programmable computers. These algorithms require multiple passes through the sample set. Each pass requires the moving of data among locations in memory and comparing operations in an arithmetic logic unit (ALU). The disadvantage of these prior techniques is that they are extremely slow compared to the subject invention.
It may further be seen that in the OS CFAR application, the sample cell or test cell is excluded from the ranking process U.S. Pat. No. 4,649,394 to Minker and Rohling discloses an OS CFAR radar in which rank ordering is proposed, but does not disclose circuitry capable of rank ordering the number set while excluding the sample or test cell from the ranking process. Such capability is also lacking in other number ranking circuits, for example, as disclosed in U.S. Pat. No. 3,927,391. The circuitry of that patent further does not output or select a particular value, but rather outputs only the rank of each member in a set of numbers. Also, according to the '391 patent, if two samples have the same value, they are given identical rank, which precludes the circuit from outputting a single member of a particular rank.